In safety-related systems and devices, an emergency-stop pushbutton can be provided for the purpose of switching the system or device. Shutdown takes place by way of a safety device or protective device.
FIG. 1 illustrates such a system in the form of a block diagram. The protective device 1, which may also be referred to as an emergency-stop device, has a multichannel input, via which it receives an emergency-stop signal from an emergency-stop pushbutton 2. Furthermore, the protective device 1 has a second input, via which a switch-on signal from an on pushbutton 3 can be received. This on pushbutton 3 is in the form of a normally open pushbutton.
The protective device 1 drives an actuator 4, for example a contactor. The actuator 4 for its part switches a load contact 5 and, at the same time, a monitoring contact 6. In the non-active state of the actuator 4, the load contact 5 is open and the monitoring contact 6 is closed.
In accordance with the European safety standard EN 954, for category 4, an automatic restart should not take place after an emergency stop on a safety device once the cause of the emergency stop has been eliminated. Thus, once an emergency-stop pushbutton has been pressed, it is first latched again for reconnection purposes. In this case, the device does not yet start. It is only when an on pushbutton is actuated that the output(s) is/are released and the device starts up.
In addition, for category 4 of this safety standard EN 954, monitoring of the actuators needs to be provided, i.e. monitoring contacts 6 of the emergency-stop device 1 or the connected actuator 4 need to be introduced into the on-pushbutton circuit. As a result, faults on the actuator 4 can be recognized. If, for example, the load contact 5 of a contactor is welded owing to an overload, the monitoring contact 6 is open even if the contactor is not activated. This means that the on-pushbutton circuit is interrupted and a reconnection is prevented.
For category 4 of the safety standard EN 954, it is also prescribed that evaluation of the on-pushbutton circuit needs to be edge-sensitive. In particular, the falling edge or both edges of a switch-on pulse need to be detected. As a result, undesired restarting of an electrical device, for example, owing to a cross-connection at the inputs or outputs of the emergency-stop device 1 can be prevented.
In the case of relatively large systems, for example a conveyer belt, two or more emergency-stop pushbuttons can be provided. In this case, the associated protective devices are connected in cascade fashion, as illustrated in FIG. 2. The fundamental design of each circuit group corresponds to that in FIG. 1.
For reasons of clarity, only two stages of a cascade are illustrated in FIG. 2, each stage having a protective device or emergency-stop device or an emergency-stop pushbutton. The first emergency-stop device 11 is connected to a two-channel emergency-stop pushbutton 12, 12′. This in turn likewise drives two actuators or contact multipliers 14 and 14′ so as to adhere to category 4 of the safety standard EN 954. The actuators 14 and 14′ actuate monitoring contacts 16 and 16′, which are connected in series and form, with an on pushbutton 13, an on-pushbutton circuit, for example from an external supply voltage source (24 V) to the emergency-stop device 11.
In addition, in each case two twin load contacts 15 and 15′ are actuated with the aid of the actuators 14 and 14′. These twin load contacts 15 and 15′ form, together with a two-channel emergency-stop pushbutton 22, 22′, an emergency-stop circuit at the second emergency-stop device 21. This likewise drives two actuators or contact multipliers 24 and 24′ which, for their part, actuate monitoring contacts 26 and 26′ and twin load contacts 25 and 25′. The monitoring contacts 26 and 26′ form, with an on pushbutton 23, the on-pushbutton circuit of the emergency-stop device 21. The cascading can be continued correspondingly as desired at the twin load contacts 25 and 25′.
The way in which this cascade circuit functions will be described below. If the emergency-stop pushbutton 12, 12′ is actuated at the first emergency-stop device 11, the actuators 14, 14′ of the emergency-stop device 11 are de-energized. The load contacts 15, 15′ of these actuators 14, 14′, which are incorporated in the emergency-stop circuit of the emergency-stop device 21, are thus also opened. As is already the case in the emergency-stop circuit of the emergency-stop device 11, this leads to an interruption in the emergency-stop circuit of the emergency-stop device 21. The actuators 24 and 24′ of the emergency-stop device 21 are therefore also switched off. As a result, all of the load circuits in the cascade are switched off.
If only the emergency-stop pushbutton 22, 22′ is actuated, only the actuators 24, 24′ of the emergency-stop device 21 are de-energized. The load monitoring contacts 26, 26′ and the twin load contacts 25, 25′ are therefore also de-energized. As a result, all of the downstream load circuits in the cascade are switched off. The actuators 14, 14′ of the emergency-stop device 11 remain active.
In order to switch the load circuits on again, the emergency-stop pushbutton 12, 12′ of the emergency-stop device 11 is latched again, i.e. the emergency-stop circuit is in a closed state. At this point in time, the load circuits are not yet complete, however. For this, the on pushbuttons 13 and 23 also need to be actuated.
If the on pushbutton 23 at the emergency-stop device 21 is actuated first, the emergency-stop device 21 does not switch its actuators 24, 24′ on since the actuators 14, 14′ of the emergency-stop device 11 still interrupt the emergency-stop circuit of the emergency-stop device 21. In order to switch the load circuits on again, initially the on pushbutton 13 of the emergency-stop device 11 therefore needs to be actuated. Thereupon, the actuators 14, 14′ of the emergency-stop device 11 are driven and the emergency-stop circuit of the emergency-stop device 21 is closed. Only now can the emergency-stop device 21 be started by actuating its on pushbutton 23. Thus, a prescribed sequence needs to be maintained for actuating the on pushbutton. However, such an inconvenient operating method for systems is unsuitable in industry.
A similar safety switching device system has been disclosed in document DE 100 11 211 A1. This safety switching device system includes two safety switching devices which correspond to one another in terms of their design and their operation. The two devices are connected one behind the other with an emergency-stop switch connected in between. In addition, a starting switch is provided on both devices.